Lignocellulosic biomass is a complex structure comprising cellulose, hemicellulose, and lignin in which cellulose and hemicellulose are bound to the lignin. Cellulose is a polymer of D-glucose with β[1-4] linkages between each of the about 500 to 10,000 glucose units. Hemicellulose is a polymer of sugars, primarily D-xylose with other pentoses and some hexoses with β[1-4] linkages derived from herbaceous materials and various hardwood species. Lignin is a complex random polyphenolic polymer.
There are a variety of widely available sources of lignocellulosic biomass including, for example, corn stover, agricultural residues (e.g., straw, corn cobs, etc.), woody materials, energy crops (e.g., sorghum, poplar, etc.), and bagasse (e.g., sugarcane). Thus, lignocellulosic biomass is a relatively inexpensive and readily available substrate for the preparation of sugars, which may be fermented to produce alcohols such as ethanol. Ethanol has a number of uses, including in fuel. For example, ethanol may be used as an additive to gasoline to boost octane, reduce pollution, and/or to partially replace gasoline and reduce crude oil requirements.
Generally, preparation of ethanol from lignocellulosic biomass involves (1) liberating cellulose and hemicellulose from lignin and/or increasing the accessibility of cellulose and hemicellulose to enzymatic hydrolysis, (2) depolymerizing carbohydrate sugars of hemicellulose and cellulose to free sugars, and (3) fermenting the sugars to ethanol.
Processes for preparation of ethanol from lignocellulosic biomass are known, but there remains an unfulfilled need for an ethanol production process that may be practiced economically on a commercial scale. For example, the need exists for ethanol production processes that provide improved ethanol yields over conventional processes and/or provide useful, improved co-products of ethanol production.